A liquid crystal display device using a liquid crystal compound has been widely utilized for a display of a watch, a calculator, a personal computer and so forth. In the liquid crystal display device, a classification based on an operating mode for liquid crystals includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode. A classification based on a driving mode in the device includes passive matrix (PM) and active matrix (AM). The passive matrix (PM) is classified into static, multiplex and so forth, and the AM is classified into thin film transistor (TFT), metal insulator metal (MIM) and so forth.
The liquid crystal display device includes a liquid crystal composition having suitable physical properties. In order to improve characteristics of the device, the composition preferably has suitable physical properties. General physical properties necessary for the liquid crystal compound being a component of the composition are as follows:                (1) being chemically stable and physically stable;        (2) having a high clearing point (clearing point: phase transition temperature between a liquid crystal phase and an isotropic phase);        (3) having a low minimum temperature of the liquid crystal phase (a nematic phase, a smectic phase or the like);        (4) having an excellent compatibility with other liquid crystal compounds; and        (5) having a small viscosity.        
As described in property (1), if a composition containing a liquid crystal compound being chemically and physically stable is used in the liquid crystal display device, a voltage holding ratio can be increased.
As described in properties (2) and (3), a composition containing a compound having the high clearing point or the low minimum temperature of the liquid crystal phase has a wide temperature range of the nematic phase. Therefore, the device can be used in a wide temperature range.
In order to develop characteristics that are difficult to be output by a single compound, the liquid crystal compound is generally used in the form of a liquid crystal composition prepared by mixing the compound with a number of other liquid crystal compounds. Accordingly, as described in property (4), the liquid crystal compound to be used in the device preferably has the good compatibility with other compounds.
Moreover, a response speed of the device is correlated with viscosity of the liquid crystal composition. Therefore, in order to manufacture a device that can respond at a high speed, a composition having the small viscosity should be used. In order to decrease the viscosity of the composition, a liquid crystal compound having the high clearing point and the small viscosity is used as a viscosity reducer. Accordingly, as described in property (5), a liquid crystal compound having the small viscosity is required.
In general, the clearing point and the viscosity of the liquid crystal compound are correlated with the number of ring structures that constitute the compound. More specifically, as the number of rings is increased, the clearing point is increased and the viscosity is also increased. Conversely, as the number of rings is decreased, the clearing point is decreased and the viscosity is also decreased. Therefore, a development has been required for a compound that has a liquid crystal phase, even without a ring structure, and has a high clearing point.
As the compound that has the liquid crystal phase even without the ring structure, a compound having a perfluoroalkyl chain has been found out so far. For example, Non-patent literatures Nos. 1 to 9 disclose straight-chain compounds (S-1) and (S-2) in which a perfluoroalkyl chain and an alkyl chain are connected. However, the compounds have a very low compatibility with other compounds, and an insufficiently high maximum temperature upon addition to a composition. Therefore, no example has been known in which the compounds are used as constituents of the liquid crystal composition for use in the liquid crystal display device.
Moreover, Patent literatures Nos. 1 and 2 disclose straight-chain compound (S-3) to straight-chain compound (S-5) that have an alkyl chain or an alkenyl chain at both terminals of a perfluoroalkyl chain. However, the compounds have no liquid crystal phase, and an insufficiently high maximum temperature upon addition to a composition. Therefore, a temperature range in which the compounds can be used in the liquid crystal display device is insufficiently wide.
